linux-hardening - Re: [PATCH v7 7/8] fortify: Make sure strlen() may still be used as a constant expression

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Message-ID: <CAKwvOd=wre3uzFVBFaOs4Oud+SobxiW_BwKXMsa5p0tEy6BsiA@mail.gmail.com>
Date:   Tue, 8 Feb 2022 15:17:13 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        George Burgess IV <gbiv@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v7 7/8] fortify: Make sure strlen() may still be used as a
 constant expression

On Tue, Feb 8, 2022 at 2:53 PM Kees Cook <keescook@...omium.org> wrote:
>
> In preparation for enabling Clang FORTIFY_SOURCE support, redefine
> strlen() as a macro that tests for being a constant expression
> so that strlen() can still be used in static initializers, which is
> lost when adding __pass_object_size and __overloadable.
>
> An example of this usage can be seen here:
>         https://lore.kernel.org/all/202201252321.dRmWZ8wW-lkp@intel.com/
>
> Notably, this constant expression feature of strlen() is not available
> for architectures that build with -ffreestanding. This means the kernel
> currently does not universally expect strlen() to be used this way, but
> since there _are_ some build configurations that depend on it, retain
> the characteristic for Clang FORTIFY_SOURCE builds too.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/fortify-string.h | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index db1ad1c1c79a..f77cf22e2d60 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -2,6 +2,8 @@
>  #ifndef _LINUX_FORTIFY_STRING_H_
>  #define _LINUX_FORTIFY_STRING_H_
>
> +#include <linux/const.h>
> +
>  #define __FORTIFY_INLINE extern __always_inline __gnu_inline
>  #define __RENAME(x) __asm__(#x)
>
> @@ -95,9 +97,16 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const p, __kernel_size_t m
>         return ret;
>  }
>
> -/* defined after fortified strnlen to reuse it. */
> +/*
> + * Defined after fortified strnlen to reuse it. However, it must still be
> + * possible for strlen() to be used on compile-time strings for use in
> + * static initializers (i.e. as a constant expression).
> + */
> +#define strlen(p)                                                      \
> +       __builtin_choose_expr(__is_constexpr(__builtin_strlen(p)),      \

Is `__is_constexpr(p) == __is_constexpr(__builtin_strlen(p))`? i.e.
can we drop the first `__builtin_strlen`? It seems redundant.

So instead, we'd have:

#define strlen(p) __builtin_choose_expr(__is_constexpr(p),
__builtin_strlen(p), __fortify_strlen(p))

Or is there some funny business where p isn't constexpr but strlen(p)
somehow is? I doubt that.  (Or is it that p is constexpr, but
strlen(p) is not?)

(Guess I'm wrong: https://godbolt.org/z/19ffz7vjx)

Ok then.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> +               __builtin_strlen(p), __fortify_strlen(p))
>  __FORTIFY_INLINE __diagnose_as(__builtin_strlen, 1)
> -__kernel_size_t strlen(const char * const p)
> +__kernel_size_t __fortify_strlen(const char * const p)
>  {
>         __kernel_size_t ret;
>         size_t p_size = __builtin_object_size(p, 1);
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers